Sputtering Processes
Over the past 30 years or so there have been numerous reviews of sputtering and sputtering processes for film deposition.
Because there are so many interactions among parameters in sputtering systems, it is impossible to separate them completely.
Typically, the target (a plate of the material to be deposited or the material from which a film is to be synthesized) is connected to a negative DC voltage supply (or an RF power supply). The substrate is the material to be coated and it faces the target. The substrate may be grounded, floating, biased, heated, cooled, or some combination of these. A gas is introduced to provide a medium in which a glow discharge can be initiated and maintained. Gas pressures ranging from a few millitorr to several tens of millitorr are used. The most common sputtering gas is argon.
When the glow discharge is started, positive ions strike the target plate and remove mainly neutral target atoms by momentum transfer, and these condense on the substrate to form thin films. There are, in addition, other particles and radiation produced at the target, all of which may affect film properties (secondary electrons and ions, desorbed gases, X-rays, and photons). The electrons and negative ions are accelerated toward the substrate platform and bombard it and the growing film. In some instances, a bias potential (usually negative) is applied to the substrate holder, so that the growing film is subject to positive ion bombardment. This is known variously as bias sputtering or ion plating.
In some cases, gases or gas mixtures other than Ar are used. Usually this involves some sort of reactive sputtering process in which a compound is synthesized by sputtering a metal target (e.g., Ti) in a reactive gas (e.g., O.sub.2 or Ar-O.sub.2 mixtures) to form a compound of the metal and the reactive gas species (e.g., TiO.sub.2).